This invention relates to a solvent sensitive, and/or pressure sensitive ink which may be printed on security documents or other document substrates. More particularly, the invention relates to an ink containing a solvent soluble, water insoluble dye, which may be combined with an encapsulated solvent and printed on a document. When exposed to a solvent or pressure, the dye in the ink composition forms a visible stain or image on the document.
Various means have been employed in the past to aid in the prevention of fraudulent alteration of security documents. One known method is to produce a covert (i.e., initially colorless) image on a security document using a self-contained coating. A typical self-contained coating comprises an encapsulated color former such as a leuco dye with a dispersed color developer. When pressure is applied to the coating, the capsules containing the leuco dye rupture and react with the color developer, causing a color change within the impacted area. If alteration of a document is attempted by rubbing the document, the pressure causes the encapsulated dye to be released to react with the color developer, producing a clear indication of the attempted alteration.
Self-contained coatings of encapsulated color formers and dispersed color developers have also been used on business forms, or other documents such as mailers where it is desirable to selectively print information on the inside or outside of a document. Such documents are typically printed using impact printing devices in which the impact of the device causes an image to develop in a coated area on the exterior or interior portion of the document.
However, such self-contained coatings suffer from a number of disadvantages. For example, premature coloration problems may occur on the document due to the presence of free color former in the coating after encapsulation which prematurely reacts with the color developer. If such a premature coloration occurs on a security document, this may result in a genuine document being dishonored as it may appear that the document has been altered. Further, yellowing of the dispersed color developer as it is exposed to air and light may also lead to confusion. Also, typical prior art self-contained coatings must be applied as low solids containing compositions which results in higher drying requirements and sheet distortion upon drying of the solvent carrier. Finally, the use of dyestuffs encapsulated in solvents is known to produce weak colored images. If higher coat weights are used to compensate for the weak color, higher costs result because of the amount of dye required.
Another known method of protection against fraudulent alteration of security documents is to incorporate small particles of a solvent soluble, but water insoluble dye into paper, such as at the head box in a paper mill, so that the dye particles become entwined with the paper fibers. The dye particles will develop a visible stain or speckle on contact with certain solvents, or ink eradicators used by persons attempting fraudulent alterations. However, those methods require incorporation of the dye at the paper making stage. Further, the speckles are not necessarily an obvious indicator of attempted alteration. A clerk or cashier may simply believe the speckles to be part of the document background.
Accordingly, there is still a need in the art for a solvent and/or pressure sensitive ink which is cost effective to prepare, free of premature coloration problems, and produces a strong colored image or stain when exposed to solvents and/or pressure. Further, there is a need for a solvent and/or pressure sensitive ink which may be applied to security documents as a means of protection against fraudulent alteration. Still further, there is a need for a self-contained pressure sensitive ink which may be printed on document substrates as a means of forming visible images on such documents.